feat: add host-side auto-updater for zero-touch API container rebuilds

When the in-container self-updater downloads a new version, it writes
trigger.json. The new systemd path unit watches for this file and runs
dashcaddy-update.sh, which backs up current API files, copies the new
ones, rebuilds the container, verifies health, and writes result.json.
Automatic rollback on build or health check failure.

Also fixes undefined `isWindows` variable in self-updater.js and adds
DASHCADDY_HOST_UPDATES_DIR env var to the installer's docker-compose
template for correct container-to-host path translation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-api/self-updater.js

@@ -17,6 +17,7 @@ const crypto = require('crypto');
 const { execSync } = require('child_process');
 const zlib = require('zlib');
 const platformPaths = require('./platform-paths');
+const isWindows = platformPaths.isWindows;
 
 const DEFAULTS = {
   CHECK_INTERVAL: 30 * 60 * 1000,  // 30 minutes